Yellowstone Supervolcano's Size Exceeds Expectations

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Beneath Yellowstone National Park lurks a partially molten plume
rising from the Earth's mantle, fueling the park's famous geysers
and hot springs, and causing the crust above to bulge and recede
in response to its forces.

Now researchers report that the source beneath the surface may be
even more massive than previously thought. Using a new technique,
they have created an image of the plume beneath Yellowstone
showing the cyclone shape stretching at a 40-degree angle to the
west at a depth of 200 miles for 400 miles east to west, as far
as the new technique can reach.

This study does not make any predictions about future eruptions,
which the USGS Yellowstone Volcano Observatory notes are of very
low probability in any given millennium, since they have been
separated in the past by 800,000 and 660,000 years.

When the Yellowstone supervolcano last erupted cataclysmically
640,000 years ago, it formed the Yellowstone Caldera, a 30 by 50
mile crater. Smaller, non-explosive eruptions have happened
since, the most recent about 70,000 years ago.

Previous estimates of the plume have used seismic images, which
measure the reflection of seismic waves from earthquakes off of
different types of materials below the surface. They reached even
deeper than the new images -- to more than 400 miles down.

The new method detects differences in electrical conductivity
generated by the different types of rocks and minerals below
Yellowstone National Park, which provides clues to what they are
made of.

Using supercomputers, the research team, led by Michael Zhdanov
of the University of Utah in Salt Lake City, created images of
the plume based on the electromagnetic measurements from 115
stations in Idaho, Montana and Wyoming.

"We see that there is a partially melted, conductive plume at
great depths starting in the mantle, and going up," Zhdanov said.

"It's a completely different technique, completely different
data," he added. "It confirms that the plume is there, but it
provides another view of the plume."

The plume's high conductivity suggests it contains high levels of
silicate rocks and perhaps briny water, he said. The observation
that the high conductivity plume is larger and angled differently
than the one found with seismic imaging suggests that the plume
of molten and partially molten rock may be surrounded by
additional liquid including briny water, Zhdanov said.

"I think it's an important finding to have a new technique to
corroborate the way that this hotspot is rising through the
mantle," said Jake Lowenstern, USGS scientist in charge of the
Yellowstone Volcano Observatory.

He noted that the finding does not indicate that the plume of
molten material is necessarily bigger than earlier seismic images
indicated, but that the plume's sphere of influence extends
further than could be seen by the other technique. "You're
looking at the effects of that plume but not necessarily the
plume itself," he said.

"It doesn't have great effect on the actual risk from the much
more shallow volcanic system," he added. While this plume
provides the heat that ultimately reaches the surface, he said,
with these new images, "you're looking at something that's way
below the actual magma chamber that's responsible for the
eruptions."

The new study will be published in an upcoming issue of the
journal Geophysical Research Letters.